Farmland-exploiting goose and swan species are expanding their ranges

By Henning Heldbjerg, Department of Ecoscience, Aarhus University, Denmark, https://ecos.au.dk/

Every winter Denmark is visited by large numbers of geese and swans from more northerly breeding sites. Many large-bodied herbivorous goose and swan species using farmland have shown rapid increases in abundance in the last 50–75 years. Increasing numbers have led to increased exploitation of agricultural crops, which has caused damage and reduction in yields and created conflicts with the farming community.

Photo by Rasmus Due Nielsen.

In a paper published in Ibis, January 2024, we compared changes in annual abundance and distribution of 12 wintering geese and swan species and tested the hypothesis that increases in national winter abundance since 2003 correlated with an expansion into formerly unoccupied winter farmland habitat. We found that nine populations showed significantly positive correlations between annual abundance and distribution range size. Five of these showed significant increases in national population abundance (Barnacle Goose, Greylag Goose, White-fronted Goose, Bewick’s Swan and Whooper Swan) between 2002/03 and 2020/21 and two have declined (Canada Goose and Taiga Bean Goose).

Species example (Pink-footed Goose Anser brachyrhynchus) showing how the available data from Denmark were collated and used. Annual winter maximum of staging/foraging birds for every site in the DOFbasen portal was extracted for each winter (A) and converted to the maximum within a 10 x 10 km square (B). For each winter, the number of squares with observations above a threshold value as percentage of all squares with any Anatidae records were calculated, and trends were calculated for each region (C). Annual national proportion of occupied squares and annual abundance estimates were compared (D) and correlated. (Copy of fig. 2 in the referred paper in Ibis).

These results support our prediction that the five key increasing goose and swan populations benefiting from farmland exploitation are showing the greatest correlation between changes in abundance and wintering range in Denmark. This implies that continued growth in abundance in these populations will lead to continued expansion in their ranges, which will increase the potential for agricultural conflict and the geographical extent of such conflicts.

We used data from bird monitoring projects undertaken by large numbers of citizen scientists. We included one systematic count of wintering waterbirds at all important waterbird sites in Denmark, coordinated by Aarhus University and with the involvement of hundreds of volunteer birdwatchers. We also included unsystematic data from DOFbasen, the largest online portal for non-systematic reporting of bird observations in Denmark (> 2 mio. records annually). All observations from each population in each winter were assigned to 10 x 10 km grid squares to describe the annual distribution of staging, wintering geese and swans. These were in turn amalgamated into seven discrete recording regions to test for regional differences.

The economic impact of geese and swans feeding on crops and throughout northwest Europe has created major conflicts. The results confirm that continued increases in some goose and swan populations are likely to result in further expansion and further conflict in new areas, which is important knowledge on which to develop future policy in finding solutions to such conflict.

All information on the 12 species is available in supporting information to the paper. The results are useful for any scientific or management interest in one or more of the species.